Lignin by-products of the black liquor residue of a kraft pulping process have been variously employed as additives in other chemical compositions. Sulfonated lignins are used as dispersants in textile dyestuff compositions and pigment printing pastes. Lignin by-products have also found use as grinding aids in oil well drilling operations, and as emulsifiers in other chemical compositions. Various methods of recovery, purification, and modification of lignin by-products as water-soluble sulfonated and sulfomethylated salts for use as additives in dyestuff compositions are described and disclosed in the following U.S. patents: U.S. Pat. No. 2,525,433; Adler et al. U.S. Pat. No. 2,680,113; U.S. Pat. No. 2,690,973; Keirstead et al. U.S. Pat. No. 3,094,515; Baisdell U.S. Pat. No. 3,158,520; U.S. Pat. No. 3,503,762; Detroit U.S. Pat. No. 3,726,850; Hintz U.S. Pat. No. 3,769,272; Falkehag et al. U.S. Pat. No. 3,841,887; Dilling et al. U.S. Pat. No. 4,001,202; Dilling U.S. Pat. No. 4,131,564; Lin U.S. Pat. No. 4,184,845; Lin U.S. Pat. No. 4,308,203; and Dilling et al. U.S. Pat. No. 4,355,996.
Generally, the lignin material of a kraft pulping process employed in paper making is recovered from the black liquor by lowering the pH of the liquor with carbon dioxide from an initial pH of about 13 to a pH of around 9.5 to precipitate the lignin material from the black liquor. The pH of the precipitated lignin slurry which may be modified, as by sulfonation, if desired, is further lowered with sulfuric acid to a pH of about 1.5 to 3, at which point the solid lignin is washed with water to remove inorganic salts and impurities therefrom. The purified lignins are then employed as additives in specified chemical compositions.
In U.S. Pat. No. 4,470,876, Beaupre' et al. teach a process for the treatment of kraft black liquor which comprises acidifying the black liquor with sulfuric acid "where the pH of the mixture is maintained at most about 5, preferably below about 4.5" (col. 2, lines 30-31), prior to separation of the lignin therefrom.
Such purified lignins have been under study for a number of years as binders in printing inks. Advantages of this type lignin as compared to other binders such as rosin, asphalt, phenolic resins, and waxes are that it is non-tacky, non-thermo-plastic, chemically inactive under printing conditions, and free of phenolic resin odor. When compared to news inks which contain an oil vehicle and no binder, lignin inks have the further advantage of being non-smudging.
One type of lignin-containing ink, which has been described, employs lignin solvents to carry the lignin which in turn binds the pigments (U.S. Pat. No. 2,525,433). Such solvents may be the glycols, glycol ethers, glycol esters, methyl ethyl ketone, and many others. The pigment weight may amount to 2 to 60% of the weight of solvent, depending on desired color and viscosity. From 1 to 5 parts of lignin per part of pigment are used. This type of ink is set either by contacting the print with water, which precipitates the lignin, or by heating to drive off the lignin solvent. Examples are as follows:
______________________________________ I Lignin 100 parts Diethylene glycol 100 Carbon black 20 Set by flowing water across printed surface. II Lignin 100 parts Ethylene glycol 235 Carbon black 25 Set by drying with radiant heat. ______________________________________
Another type of ink designed for high-speed printing of better quality than attainable with usual news inks has also been reported (U.S. Pat. No. 2,449,230). This ink employs water as well as a lignin solvent and dries by breaking upon contact with the surface to be printed, whereupon the water and solvent are released and taken up by the sheet. This permits very rapid printing and immediate piling of the printed sheets without offset. A typical formula for a news ink of this type is:
Lignin: 20 parts PA1 Water: 51 PA1 2-amino-2methyl-1,3-propanediol: 10 PA1 Paraformaldehyde: 4.5 PA1 Diethylene glycol monobutyl ether: 1.8 PA1 Peacock blue: 12.7
Numerous other formulations of varying properties for magazines, rotogravure, hard papers, fabrics, etc., are described, which teaching is incorporated by reference herein.
Recent attempts have been made to use such purified lignin materials in water-based carbon black printing compositions both as a grinding aid and as an adhesive to bind the carbon black to the substrate in printing operations. To render the purified lignins water-soluble, an organic amine, such as monoethanolamine (MEA), must be added to an aqueous lignin slurry to form a water-soluble lignin amine salt for addition to the printing composition. However, this proposed use of lignins in such water-based printing compositions has been rendered impractical because the lignin exhibits poor solubility and poor stability in aqueous solution with MEA. In particular, it has been found that the poor water solubility of the amine lignin salts prepared as described results in sludging and viscosity variations with consequent reduced effectiveness of the formulated printing compositions.